Thermolabile exonucleases

ABSTRACT

The invention provides an exonuclease or an enzymatically active fragment thereof, said exonuclease having the amino acid sequence of SEQ ID NO:1 or an amino acid sequence which is at least about 50% identical thereto, wherein said exonuclease or enzymatically active fragment thereof (i) is substantially irreversibly inactivated by heating at a temperature of about 55° C. for 10 minutes in a buffer consisting of 10 mM Tris-HCl, pH 8.5 at 25° C., 50 mM KCl and 5 mM MgCl 2 ; (ii) is substantially specific for single stranded DNA; and (iii) has a 3′-5′ exonuclease activity. The invention further provides a method of removing single stranded DNA from a sample, a method of nucleic acid amplification, a method of reverse transcription and a method of nucleic acid sequence analysis in which the exonuclease or enzymatically active fragment thereof is used. The invention still further provides nucleic acids encoding said exonuclease or an enzymatically active fragment thereof and kits or compositions comprising the same.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent application Ser. No. 15/504,507, filed 16 Feb. 2017, which in turn is a national stage filing under 35 U.S.C. § 371 of PCT/EP2015/001703, filed on 19 Aug. 2015, which in turn claims the benefit of priority to and the benefit of GB Application No. 1414745.8, filed 19 Aug. 2014. Each application is incorporated herein by reference in its entirety.

SEQUENCE LISTING SUBMISSION

The present application is being filed along with a Sequence Listing in electronic format. The Sequence Listing is entitled 3403-128SequenceListing.txt, created on 30 Jul. 2010, and is 68 kb in size. The information in the electronic format of the Sequence Listing is incorporated herein by reference in its entirety.

The present invention relates to thermolabile exonucleases and the use of the same to remove single stranded DNA from samples containing biological molecules, in particular the products of nucleic acid amplification or reverse transcription reactions (e.g. double stranded DNA, RNA and DNA:RNA duplexes). The invention may therefore be viewed particularly as relating to the refinement of samples containing double stranded DNA, RNA and/or DNA:RNA duplexes by removing single stranded DNA. More specifically the invention relates to the removal of excess oligodeoxyribonucleotide primers from the products of nucleic acid amplification or reverse transcription reactions. Removal of excess oligodeoxyribonucleotide primers from the products of nucleic acid amplification reactions may enhance the downstream sequence analysis of such products as interference from excess oligodeoxyribonucleotide primers is reduced. The present invention therefore further relates to methods for optimising the sequence analysis of the products of a nucleic acid amplification or reverse transcription reactions, e.g. by nucleic acid sequencing or oligonucleotide probe arrays. Methods for optimising the sequence analysis of nucleic acids that have not been obtained from a nucleic acid amplification reaction are also provided.

Nucleases are enzymes that break the phosphodiester bonds in the sugar-phosphate backbone of DNA or RNA polymers. Nucleases are a very diverse group of enzymes. The mode of action can be either highly specific or very general depending upon their target function. Nucleases may prefer single stranded (ss) polymers or double stranded (ds) polymers. Some nucleases cleave at specific nucleotide sequences (e.g. restriction endonucleases), whereas others cleave at positions in the polymers independent of nucleotide sequence. In the cell, nucleases have a variety of functions and are involved in DNA replication, recombination, mutation, transcription and repair in addition to breaking down redundant pieces of RNA and DNA. Nucleases may also serve a role in host defence mechanisms.

All nucleases may be divided into three major classes based on whether their catalytic mechanism involves two, one or no metal ions (the two-metal-ion dependent, one-metal-ion-dependent and metal independent nuclease superfamilies). Each of these classes includes many different families and superfamilies. For more details about nuclease classification see Yang, W., 2011, “Nucleases: diversity of structure, function and mechanism.” Q Rev Biophys 44(1): 1-93.

Based on substrate preference nucleases may be classified as deoxyribonucleases (DNases) or ribonucleases (RNases), i.e. enzymes that cleave the phosphodiester bonds of either DNA or RNA, respectively. Based on the positions of the cleaved bonds within the DNA or RNA polymers, nucleases may be further classified as endonucleases or exonucleases. The endonucleases cleave phosphodiester bonds of DNA or RNA at positions within the polymer, whereas exonucleases are involved in trimming the ends of RNA and DNA polymers, cleaving the outermost phosphodiester bond in a chain. Exonucleases can be further divided into two groups by the 5′ to 3′ versus 3′ to 5′ polarity. Nucleases may also display specificity between single- and double-stranded nucleic acids. A particular nuclease may be strictly single-strand-specific, single-strand-preferential, strictly double-strand-specific, double-strand-preferential or a nuclease that cleaves both.

A number of exonuclease enzymes are known, and among them, the Escherichia coli enzymes are the most well-characterised ones. A common feature for exonucleases is their high processivity degrading up to a 1000 nucleotides in a single binding event releasing mononucleotides. Exonuclease I (ExoI), Exonuclease VII (ExoVII) and RecJ exonuclease are all reported to be ssDNA-specific exonucleases involved in DNA repair. However, their polarity of action is different. ExoI possesses 3′ to 5′ exonuclease activity while RecJ possesses only 5′ to 3′ activity. In contrast to the other exonucleases, ExoVII possesses both 5′ to 3′ and 3′ to 5′ exonuclease activities.

The determination of the sequence of nucleotides in nucleic acids, e.g. DNA and RNA, has become an important goal in modern molecular biology. By analysing such sequences information on the source of the nucleic acid can be obtained. For instance, the nucleotide sequence of certain evolutionarily variable genetic elements can provide an indication as to the identity of the organism from which it is derived. Accordingly, detecting nucleic acid carrying a nucleotide sequence characteristic of a particular microorganism in a sample can indicate the presence of the microorganism in the sample or even quantify the amounts of such organisms in the sample. Sequence analysis can also assist in the taxonomic classification of higher organisms, which may be important in technical fields such as agriculture and veterinary science. In humans, nucleotide sequence analysis can identify individuals and their lineage, thus having forensic applications, and can identify medically or physiologically relevant genotypes, e.g. mutations. The sequencing of the RNA transcripts of a target cell or group of cells (e.g. a tissue, a tumour or a culture) can yield information on the transcriptome of the target, which in turn can have numerous applications in the medical and scientific fields. Nucleic acid based identity tags carrying unique nucleotide sequences are also available, the detection of which requires the analysis of the nucleotide sequence of the tag. In other representative applications the skilled person may wish to ascertain the nucleotide sequence of a nucleic acid with which he/she is working, perhaps either to confirm a manipulation has succeeded or to understand the make-up of a novel molecule.

Nucleic acid sequence analysis may take the form of a sequencing technique. The Sanger dideoxynucleotide sequencing method is a well-known and widely used technique for sequencing nucleic acids. However more recently the so-called “next generation” or “second generation” sequencing approaches (in reference to the Sanger dideoxynucleotide method as the “first generation” approach) have become widespread. These newer techniques are characterised by high throughputs, e.g. as a consequence of the use of parallel, e.g. massively parallel sequencing reactions, or through less time-consuming steps. Various high throughput sequencing methods provide single molecule sequencing and employ techniques such as pyrosequencing, reversible terminator sequencing, cleavable probe sequencing by ligation, non-cleavable probe sequencing by ligation, DNA nanoballs, and real-time single molecule sequencing.

Nucleic acid sequence analysis may also take the form of an oligonucleotide hybridisation probe based approach in which the presence of a target nucleotide sequence is confirmed by detecting a specific hybridisation event between a probe and its target. In these approaches the oligonucleotide probe is often provided as part of a wider array, e.g. an immobilised nucleic acid microarray.

Further approaches are available and may be developed in the future, but as a common theme it is typical, albeit not essential, that each are performed on nucleic acid that has been amplified in a nucleic acid amplification reaction or synthesised by a reverse transcription (RT) reaction. Amplification is typically required to ensure that there is sufficient nucleic acid sample for the sequence analysis. Such techniques include the polymerase chain reactions (PCRs), ligase amplification reaction (LAR; also known as ligase chain reaction (LCR)), strand displacement amplification (SDA), nucleic acid sequence based amplification (NASBA; also known as 3SR (Self-Sustaining Sequence Replication)) and may be preceded by a reverse transcription reaction. These amplification techniques and reverse transcription techniques often result in the presence of non-target single stranded DNA in the final product, mainly because an excess of single stranded oligodeoxyribonucleotide primer is often supplied initially, but single stranded DNA amplicons may also arise when polymerisation is incomplete. Such single stranded DNA interfere with the sequence analysis of the amplification product by competing with the other reagents, e.g. oligonucleotide probes, and undergoing sequencing themselves, thereby contaminating the sequencing information outputted by the reaction. This potentially lowers the sensitivity and accuracy of the analysis.

To mitigate such interference the product of a nucleic acid amplification reaction or a reverse transcription reaction may be treated with an exonuclease to degrade single stranded DNA (e.g. unincorporated primers). It is also possible to include a treatment to effect the dephosphorylation of any unincorporated NTPs (e.g. dNTPs), e.g. a treatment with an alkaline phosphatase, for instance the heat-labile shrimp alkaline phosphatase (SAP). Today exonuclease I from E. coli (ExoI) is the most commonly used exonuclease in such reactions. The exonuclease I reaction is typically performed by adding the enzyme to the product of the nucleic acid amplification or the reverse transcription reaction and incubating for 15 min at 37° C. However, to prevent the exonuclease from interfering with downstream processes, e.g. sequence analysis, it is usually necessary to inactivate the enzyme. With the commonly used exonuclease I from E. coli, inactivation requires incubation of the enzyme at 80° C. for 15-20 minutes. The long inactivation time and relatively high inactivation temperature makes the process time consuming and relatively harsh on the sample of interest.

RecJ and ExoVII have also been proposed for such a use, but are not commonly used because RecJ has a low specific activity and ExoVII consists of two different polypeptide chains. The recommended inactivation conditions for RecJ involve a 20 min incubation at 65° C. and for ExoVII a 10 minutes incubation at 95° C.

There is a need therefore to provide an enzyme capable of specifically degrading single stranded DNA in under 15 minutes and/or which may be essentially irreversibly inactivated at a temperature below 65° C. and/or in less than 15-20 minutes. Such a thermolabile (also interchangeably referred to herein as a heat-labile (HL)) enzyme would improve molecular biology techniques by making them significantly more time efficient and gentler on the sample molecule of interest.

It may be desirable to remove single stranded DNA from a sample in other contexts. For instance, to digest the single stranded DNA to which a nucleic acid binding protein is bound. In these contexts an enzyme capable of specifically degrading single stranded DNA in less than 15 minutes and/or which may be essentially irreversibly inactivated at a temperature below 65° C. and/or in less than 15-20 minutes would be advantageous over exonuclease I, RecJ and ExoVII from E. coli for the same reasons of time efficiency and gentle processing discussed above.

It has now been found that homologs of the E. coli sbcB gene (which encodes for E. coli exonuclease I) obtained from species of the genera Shewanella, Halomonas, Vibrio, Psychromonas and Moritella found in cold water niches, e.g. Moritella viscosa and Vibrio wodanis surprisingly have these advantageous properties.

Therefore, in a first aspect there is provided an exonuclease or an enzymatically active fragment thereof, said exonuclease having the amino acid sequence of SEQ ID NO:1 or an amino acid sequence which is at least about 50% identical thereto, wherein said exonuclease or enzymatically active fragment thereof

(i) is substantially irreversibly inactivated by heating at a temperature of about 55° C. for 10 minutes in a buffer consisting of 10 mM Tris-HCl, pH 8.5 at 25° C., 50 mM KCl and 5 mM MgCl₂;

(ii) is substantially specific for single stranded DNA; and

(iii) has a 3′-5′ exonuclease activity.

By “at least about 50%” it is meant that the sequence identity may be at least 49%, 49.5% or 49.9%. In preferred embodiments the exonuclease of the invention has an amino acid sequence which is at least 60%, preferably at least 70%, 80%, 85%, 90% or 95%, e.g. at least 98% or 99% or 99.5%, identical to SEQ ID NO:1. In other embodiments the exonuclease consists of the amino acid sequence of SEQ ID No 1. Enzymatically active fragments thereof are also provided.

An exonuclease having an amino acid sequence which is at least 50% identical to SEQ ID NO:1 may be obtained from a prokaryotic organism found in cold water niches. By “prokaryote” it is meant any organism that lacks a cell nucleus, i.e. any organism from the domains Bacteria and Archaea. Preferably the organism is a bacterium. Preferably the organism is not a eukaryote, e.g. an organism classified in the taxonomic kingdoms Animalia, Plantae, Fungi or Protista. More preferably the organism is selected from the genera Shewanella, Halomonas, Vibrio, Psychromonas and Moritella.

In certain embodiments an exonuclease having an amino acid sequence which is at least about 50% identical to SEQ ID NO:1 may be selected from SEQ ID NO:2 (the SbcB homolog from Halomonas sp.), SEQ ID NO:3 (the SbcB homolog from Vibrio wodanis), SEQ ID NO:4 (the SbcB homolog from Psychromonas sp.) or SEQ ID NO:5 (the SbcB homolog from Moritella viscosa).

Thus, in another aspect of the invention there is provided an exonuclease or an enzymatically active fragment thereof, said exonuclease having an amino acid sequence selected from:

-   -   (a) SEQ ID NO:1 or an amino acid sequence which is at least 65%         identical thereto,     -   (b) SEQ ID NO:2 or an amino acid sequence which is at least 65%         identical thereto,     -   (c) SEQ ID NO:3 or an amino acid sequence which is at least 65%         identical thereto,     -   (d) SEQ ID NO:4 or an amino acid sequence which is at least 65%         identical thereto, or     -   (e) SEQ ID NO:5 or an amino acid sequence which is at least 65%         identical thereto,         wherein said exonuclease or enzymatically active fragment         thereof

(i) is substantially irreversibly inactivated by heating at a temperature of about 55° C. for 10 minutes in a buffer consisting of 10 mM Tris-HCl, pH 8.5 at 25° C., 50 mM KCl and 5 mM MgCl₂;

(ii) is substantially specific for single stranded DNA, and

(iii) has a 3′-5′ exonuclease activity.

In preferred embodiments of this aspect of the invention, the exonuclease has an amino acid sequence which is at least 70%, preferably at least 80%, 85%, 90% or 95%, e.g. at least 98% or 99% or 99.5%, identical to SEQ ID NOs:1, 2, 3, 4 or 5. In other embodiments the exonuclease consists of an amino acid sequence selected from the group consisting of SEQ ID NOs:1, 2, 3, 4 and 5. Enzymatically active fragments thereof are also provided.

Percentage sequence identity according to the invention can be calculated using any of the widely available algorithms, e.g. using the Clustal W2 multiple sequence alignment program (www.ebi.ac.uk/Tools/clustalW2) using default parameters (DNA Gap Open Penalty=15.0; DNA Gap Extension Penalty=6.66; DNA Matrix=Identity; Protein Gap Open Penalty=10.0; Protein Gap Extension Penalty=0.2; Protein matrix=Gonnet; Protein/DNA ENDGAP=−1; Protein/DNA GAPDIST=4).

Variants of the abovementioned SEQ ID NOs include amino acid sequences in which one or more amino acids of said SEQ ID NOs have undergone conservative substitution or have been replaced with a modified version of said one or more amino acids or an amino acid which is not naturally occurring, e.g. D isomers of said one or more amino acids. Preferably such substitutions and modifications are silent substitutions and modifications in that the modified forms of the exonucleases of the invention have the same enzymatic and inactivation characteristics as the unmodified forms.

An exonuclease is an enzyme capable of cleaving a nucleotide from one or more termini of a polynucleotide chain, without nucleotide sequence specificity, by hydrolysing an internucleotide phosphodiester bond. Typically exonucleases cleave nucleotides from either the 5′ terminus (and so are characterised as 5′-3′ exonucleases or as having 5′-3′ exonuclease activity) or the 3′ terminus (and so are characterised as 3′-5′ exonucleases or as having 3′-5′ exonuclease activity). In accordance with the invention the exonucleases are 3′-5′ exonucleases. In some embodiments the exonucleases of the invention have substantially no 5′-3′ exonuclease activity against single stranded DNA, by which it is meant that, at concentrations of 0.1 to 1.0 U/μl in a buffer consisting of 10 mM Tris-HCl, pH 8.5 at 25° C., 50 mM KCl and 5 mM MgCl₂, the exonucleases of the invention display little, negligible or essentially no 5′-3′ exonuclease activity against single stranded DNA over the course of a 1 hour incubation. Expressed numerically, less than 10%, e.g. less than 5%, 4%, 3%, 2% or 1%, of a single stranded DNA substrate (e.g. about 5 pmol of said substrate, which may for example be an oligodeoxyribonucleotide) under such conditions will be degraded in a 5′-3′ direction. Preferably, there will be no detectable 5′-3′ exonuclease activity at such concentrations.

The skilled person would be able to devise a suitable assay to measure relative 5′-3′ and 3′-5′ exonuclease activities. For instance the gel-based exonuclease assay described in the Examples uses a 5′ (FAM) labelled single stranded oligodeoxyribonucleotide to monitor degradation from the 3′ terminus as such an activity will yield detectable fragments of varying length shortened from the 3′ terminus. Degradation from the 5′ terminus will yield only single labelled nucleotides. To confirm these results the same assay may alternatively be performed with a 3′ labelled single stranded DNA substrate and the opposite gel pattern should be observed.

By “substantially specific for single stranded DNA” it is meant that the activity of the exonuclease of the invention against double stranded DNA is equal to or less than 15% of the activity against an equivalent amount of single stranded DNA under the same conditions, e.g. an enzyme concentration of about 0.1 U/μl and about 5 pmol of nucleic acid substrate in a buffer consisting of 10 mM Tris-HCl, pH 8.5 at 25° C., 50 mM KCl and 5 mM MgCl₂ with an incubation time of 10 minutes or less and an incubation temperature of about 30° C. In other embodiments the activity of the exonuclease of the invention against double stranded DNA is equal to or less than 10%, e.g. equal to or less than 8%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.2%, 0.1% or 0.05%, of the activity against an equivalent amount of single stranded DNA under the same conditions.

In further more specific embodiments “substantially specific for single stranded DNA” also means that the activity of the exonuclease of the invention against single stranded RNA is equal to or less than 15% of the activity against an equivalent amount of single stranded DNA under the same conditions, e.g. an enzyme concentration of about 0.1 U/μl and about 5 pmol of nucleic acid substrate in a buffer consisting of 10 mM Tris-HCl, pH 8.5 at 25° C., 50 mM KCl and 5 mM MgCl₂ with an incubation time of 10 minutes or less and an incubation temperature of about 30° C. In other embodiments, the activity of the exonuclease of the invention against single stranded RNA is equal to or less than 10%, e.g. equal to less than 8%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.2%, 0.1% or 0.05%, of the activity against an equivalent amount of single stranded DNA under the same conditions.

In certain embodiments by “substantially specific for single stranded DNA” it is meant that the exonuclease of the invention degrades single stranded DNA but at concentrations of about 0.1 U/μl in a buffer consisting of 10 mM Tris-HCl, pH 8.5 at 25° C., 50 mM KCl and 5 mM MgCl₂, there is little, negligible or essentially no detectable degradation of about 5 pmol of a suitable double stranded DNA substrate (e.g. a double stranded oligodeoxyribonucleotide) over the course of an about 10 minute incubation. Expressed numerically, equal to or less than 15%, e.g. equal to or less than 10%, 8%, 5%, 4%, 3%, 2% or 1%, of the suitable double stranded DNA substrate will be degraded under such conditions. Preferably, there will be no detectable degradation of double stranded DNA substrates under such concentrations.

In further more specific embodiments “substantially specific for single stranded DNA” also means that the exonuclease of the invention degrades single stranded DNA but at concentrations of about 0.1 U/μl in a buffer consisting of 10 mM Tris-HCl, pH 8.5 at 25° C., 50 mM KCl and 5 mM MgCl₂, there is little, negligible or essentially no detectable degradation of about 5 pmol of a suitable single stranded RNA substrate (e.g. a single stranded oligoribonucleotide) over the course of an about 10 minute incubation. Expressed numerically, equal to or less than 15%, e.g. equal to or less than 10%, 8%, 5%, 4%, 3%, 2% or 1%, of the suitable single stranded RNA substrate will be degraded under such conditions. Preferably, there will be no detectable degradation of single stranded RNA substrates under such concentrations.

In certain embodiments “substantially specific for single stranded DNA” also means that the exonucleases of the invention degrade single stranded DNA but at concentrations of 0.1 to 1.0 U/μl (preferably 0.1 U/μl) in a buffer consisting of 10 mM Tris-HCl, pH 8.5 at 25° C., 50 mM KCl and 5 mM MgCl₂, there is little, negligible or essentially no detectable degradation of non-single stranded DNA nucleic acid substrates over the course of a 1 hour incubation. As used herein, the term “non-single stranded DNA nucleic acid substrates”, in some embodiments, refers both to double stranded nucleic acids, e.g. double stranded DNA, and single stranded non-DNA nucleic acid, although in preferred embodiments the term refers only to double stranded nucleic acids, e.g. double stranded DNA. Expressed numerically, less than 10%, e.g. less than 5%, 4%, 3%, 2% or 1%, of (e.g. about 5 pmol of) a suitable non-single stranded DNA substrate (i.e. double stranded nucleic acid and/or single stranded non-DNA nucleic acid) under such conditions will be degraded. Preferably, there will be no detectable degradation of non-single stranded DNA nucleic acid substrates (i.e. double stranded nucleic acid and/or single stranded non-DNA nucleic acid) at such concentrations.

The skilled person would easily be able to devise an experiment to make a comparison of relative nuclease activity towards single and double stranded nucleic acid. For instance, an exonuclease under test may be incubated with two samples of a radioactively labelled PCR product (e.g. about 5 pmol of said PCT product), one in which the product has been denatured (i.e. single stranded) and the other in which the product is not denatured (i.e. double stranded) in a buffer consisting of 10 mM Tris-HCl, pH 8.5 at 25° C., 50 mM KCl and 5 mM MgCl₂ at a concentration of 0.1 to 1.0 U/μl (preferably 0.1 U/μl) for 10 minutes. The release of acid soluble nucleotides can then be analysed as described in Examples 8 and 9.

Alternatively, an exonuclease under test may be incubated with the abovementioned samples of PCR product (e.g. about 5 pmol of said PCT product) in a buffer consisting of 10 mM Tris-HCl, pH 8.5 at 25° C., 50 mM KCl and 5 mM MgCl₂ at a concentration of 0.1 to 1.0 U/μl (preferably 0.1 U/μl) for one hour and then products separated on a suitable, electrophoresis gel e.g. agarose. Activity against single stranded and/or double stranded nucleic acid will be observable by the position of the bands relative to untreated controls.

Another approach measures the increase in fluorescence from oligonucleotides labelled with the fluorophore FAM (fluorescein) at the 5′ terminus and with TAMRA at the 3′ terminus. The emitted light from FAM is absorbed (quenched) by TAMRA when the two labels are in proximity. The cleavage of the oligonucleotide by the exonuclease under test results in the separation of FAM from TAMRA and an increase in fluorescence from FAM that can be measured in a fluorimeter with excitation wavelength 485 nm and emission wavelength 520 nm. A double stranded substrate can be prepared by mixing the labelled oligonucleotide with a second oligonucleotide that is complementary to the labelled oligonucleotide. Of course other suitable fluorophore pairs may similarly be used. Example 7 describes a suitable assay in greater detail.

Included within the term DNA are modifications thereof which retain a phosphodiester linked deoxyribo-phosphate backbone. Commonly encountered examples of single stranded DNA include oligodeoxyribonucleotide primers and oligodeoxyribonucleotide probes and DNA aptamers. Nucleic identification tags and labels may also be single stranded DNA. Single stranded DNA also arises during reverse transcription, and upon duplex unwinding during DNA replication and DNA transcription. In nucleic acid amplification reactions and reverse transcription reactions single stranded DNA may arise from partially extended primers and any excess of primers that have not been extended and integrated into completely synthesised duplexes.

In further embodiments the exonucleases of the invention have substantially no endonuclease activity. By “substantially no endonuclease activity” it is meant that, at concentrations of 0.1 to 1.0 U/μl in a buffer consisting of 10 mM Tris-HCl, pH 8.5 at 25° C., 50 mM KCl and 5 mM MgCl₂, the exonuclease of the invention displays little, negligible or essentially no nuclease activity against a circular single stranded nucleic acid or circular double stranded nucleic acid over the course of a 1 hour incubation. Expressed numerically, less than 10%, e.g. less than 5%, 4%, 3%, 2% or 1%, of (e.g. about 5 pmol of) a single or double stranded nucleic acid substrate will be fragmented (e.g. into oligonucleotides) under such conditions. Preferably, there will be no detectable endonuclease activity at such concentrations.

Enzymatically active fragments and variants of SEQ ID Nos. 1-5 display at least 70%, preferably at least 85%, more preferably at least 95% and most preferably at least 99% of the enzymatic function of the enzymes of SEQ ID Nos. 1-5, respectively. As discussed elsewhere, the activity of an exonuclease can be assessed easily using routine techniques.

In the following discussion, a reference to an exonuclease of the invention is also a reference to an enzymatically active fragment thereof, unless context dictates otherwise.

By “substantially irreversibly inactivated” is meant that on heating to the specified temperature for the specified time and under the specified buffer conditions, the enzyme is at least 90% inactivated, preferably 95%, 98%, 99%, 99.5% or 99.9% inactivated. Percentage inactivation can be conveniently estimated by incubating a suitably labelled (e.g. a 5′ FAM labelled) single stranded DNA sample (e.g. a standard PCR primer, for instance a deoxyribonucleic acid primer having the nucleotide sequence of SEQ ID NO:21—GCTAACTACCACCTGATTAC) for 30 minutes either with an inactivated exonuclease or with a non-inactivated exonuclease in a suitable buffer (e.g. Tris, HEPES, PBS) at a suitable pH (e.g. pH 7.5) at a suitable temperature (e.g. 30° C.) and in the presence of Mg²⁺ (e.g. 5 mM); separating the reaction products on a suitable gel (e.g. acrylamide/urea gel) by electrophoresis and measuring the relative intensities of fluorescence of the DNA bands under UV light e.g. as shown in Examples 5 and 6. Alternative methods could be devised by the skilled person to measure relative activities of inactivated and non-inactivated exonuclease, in particular a buffer consisting of 10 mM Tris-HCl, pH 8.5 at 25° C., 50 mM KCl and 5 mM MgCl₂ may be used.

Even when the temperature of the reaction mixture returns to room temperature, the exonucleases of the invention do not regain activity, i.e. there is substantially no residual activity; specifically, less than 10%, preferably less than 5%, 2%, 1%, 0.5% or 0.1%, most preferably no detectable exonuclease activity remains.

Substantially irreversible inactivation occurs within 10 minutes of incubation in the specified buffer conditions at a temperature of at or about 55° C., e.g. 53 to 57° C. For example in 7, 8 or 9 minutes incubation at about 55° C. As shown in Example 5, in other embodiments the exonuclease of the invention is substantially irreversibly inactivated in the specified buffer conditions at a temperature of at or about 55° C., e.g. 53 to 57° C., within 5 minutes, for example in 2, 3 or 4 minutes. In further embodiments the exonuclease of the invention is substantially irreversibly inactivated in the specified buffer conditions at a temperature of at or about 50° C., e.g. 48 to 53° C., within 10 minutes, for example in 7, 8 or 9 minutes. In still further embodiments the exonuclease of the invention is substantially irreversibly inactivated in the specified buffer conditions at a temperature of at or about 50° C., e.g. 48 to 53° C., within 5 minutes, for example in 2, 3 or 4 minutes. The exonucleases represented by SEQ ID Nos.1 and 4 and variants thereof are examples of these latter embodiments.

In other embodiments substantially irreversible inactivation of the exonuclease of the invention occurs within 1 minute of incubation in the specified buffer conditions at a temperature of at or about 80° C., e.g. 70 to 90° C., 75 to 85° C., 78 to 82° C. or 79 to 81° C. For example in an incubation of at least about 30, 40, 50 or 55 seconds at about 80° C.

When in use, the exonuclease of the invention may be substantially irreversibly inactivated at lower temperatures or over shorter time periods depending on the conditions in which the enzyme is being used, e.g. at 55° C. for 5 minutes or at 50° C. for 5 to 15 minutes, e.g. 10 to 15 minutes but, in accordance with the invention, heating for 10 minutes at about 55° C. in the specified buffer conditions must be sufficient to substantially irreversibly inactivate the enzyme. It will be readily apparent to the skilled person that adjustments to one of these two parameters can be compensated for by adjusting the other. For instance increasing the inactivation temperature might permit the duration of incubation to be reduced. Conversely, increasing the duration of incubation might permit a lower inactivation temperature to be used. Of course, as is also readily apparent to the skilled person and shown in the Examples, when the exonucleases of the invention are used in the methods of the invention, durations of incubation longer than ten minutes may be used and inactivation temperatures greater than about 55° C. may be used, if practical (e.g. inactivation could take place at 80° C. for 1 minute, 60° C. for 5 to 10 minutes, 55° C. for 15 minutes or 50° C. for 15 minutes). However, to be in accordance with the invention, an exonuclease must show substantial inactivation if incubated at a temperature of at or about 55° C. for 10 minutes in the specified buffer conditions.

Inactivation temperatures and times for an exonuclease of the invention should be assessed by incubating the exonuclease in buffer consisting of 10 mM Tris-HCl, pH 8.5 at 25° C., 50 mM KCl and 5 mM MgCl₂. The exonuclease should be present at about 0.1 to 1.0 U/μl, preferably 0.1 to 1.5 U/μl, 0.1 to 5 U/μl or 0.1 U/μl to 10 U/μl.

In most preferred embodiments an exonuclease of the invention may have (or consist of) an amino acid sequence of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, or SEQ ID NO:5.

In another aspect of the invention there is provided an exonuclease having (or consisting of) an amino acid sequence of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, or SEQ ID NO:5. Being defined entirely by amino acid sequence means the particular functional features described above do not necessarily apply to this aspect. Nevertheless, as shown in the Examples, each of these specific exonucleases finds utility in the methods of the invention described herein.

The exonucleases of the invention may be provided in a modified form, e.g. as a fusion protein with an amino acid motif useful in a process for the isolation, solubilisation and/or purification or identification of the exonucleases. Such amino acid motifs (also known as protein tags) include, but are not limited to polyhistidine (His) tags. Examples of polyhistidine tagged exonucleases of the invention are recited in SEQ ID NO:11 to 15. Such enzymes form a further aspect of the invention.

Further modifications include the introduction of small chemical groups to available atoms of the polypeptide, e.g. protecting groups for the N and C termini or the R-groups of non-essential amino acid residues within the polypeptide. In other embodiments the exonucleases of the invention may be provided immobilised on a solid support, e.g. a solid support selected from particles, pellets, beads, sheets, gels, filters, membranes, fibres, capillaries, chips, micro titre strips, slides, tubes, plates or wells etc. Preferably the support is magnetic (preferably paramagnetic or superparamagnetic) e.g. magnetic particles, for instance magnetic beads and pellets. Still further modified forms include dimers or trimers of the exonucleases of the invention. Such entities may be homogeneous or heterogeneous in their monomer composition.

The invention also provides nucleic acid molecules encoding the exonucleases of the invention and enzymatically fragments thereof. Nucleotide sequences corresponding to the amino acid sequences of SEQ ID NOs:1 to 5 are disclosed in SEQ ID NOs:6 to 10, respectively, and the nucleic acids of the invention may comprise these nucleotide sequences. Nucleotide sequences corresponding to the amino acid sequences of SEQ ID NOs:11 to 15 are disclosed in SEQ ID NOs:16 to 20, respectively, and the nucleic acids of the invention may comprise these nucleotide sequences. Degeneracy of the genetic code means that each of SEQ ID NOs:6 to 10 and 16 to 20 are each only one of many possible nucleotide sequences encoding the amino acid sequences of SEQ ID NOs:1 to 5 and 11 to 15, respectively. Accordingly, the invention extends to nucleic acid molecules comprising nucleotide sequences which are degenerate versions of SEQ ID NOs:6 to 10 and 16 to 20. The nucleic acid molecules of the invention may be nucleic acid vectors, e.g. cloning vectors or expression vectors. Preferred vectors are plasmids compatible with bacterial and/or yeast cells.

The enzymatic activities and inactivation characteristics of the exonucleases of the invention make such enzymes especially suited for the removal of single stranded DNA from samples containing biological macromolecules, in particular, the products of a nucleic acid amplification or reverse transcription reactions, e.g. double stranded DNA, RNA and DNA:RNA duplexes.

Thus, in a further aspect there is provided a method of removing single stranded DNA from a sample, preferably a sample of biological macromolecules, said method comprising contacting the sample with an exonuclease as defined above.

The exonucleases of the invention are thus used to degrade single stranded DNA present in the sample. In particular, the method involves contacting the sample with an exonuclease of the invention under conditions which permit the digestion of at least a portion of the single stranded DNA present in the sample and then optionally heating the sample to inactivate said exonuclease. These steps of digestion and inactivation will typically be incubations and are described herein, in particular in the Examples. Suitable incubation conditions to achieve digestion of single stranded DNA in a sample are known in the art and may conveniently comprise incubation at 10 to 45° C., e.g. at or around 20 to 40° C. or 30° C. for 1 to 30 minutes, e.g. 1 to 20 minutes, 1 to 15 minutes or 1 to 10 minutes, preferably around 5 minutes or less. If a temperature at the higher end of these ranges is used, the duration of incubation may be at the lower end of these ranges, and vice versa. Inactivation conditions may be based on the discussion of such parameters given above.

By “removing single stranded DNA” it is meant that the amount of single stranded DNA in the sample is reduced to some extent. Encompassed are embodiments in which the amount of single stranded DNA is reduced below detectable levels, as well as embodiments in which the amount of single stranded DNA is reduced to a smaller but still detectable amount. Preferably the amount of single stranded DNA is reduced sufficiently to improve the quality of the sample as defined by the relevant context, e.g. its use in a nucleic acid sequence analysis reaction. Expressed numerically the amount of single stranded DNA in a sample may be reduced by at least 10% e.g. by at least, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 85%, 90%, 95% or 99% or by 100%. In practical terms, to remove single stranded DNA from a sample, or to reduce the amount of single stranded DNA in a sample, in accordance with the invention, is to degrade at least a portion of the single stranded DNA present in a sample. The term “at least a portion” should be construed in line with the foregoing.

In a preferred embodiment, the sample is a preparation containing a nucleic acid of interest (e.g. DNA, RNA, PNA), e.g. a double stranded nucleic acid or a DNA:RNA duplex; a protein of interest, for example a recombinantly produced protein of interest, e.g. an enzyme; a carbohydrate polymer; or a lipid. Alternatively the protein of interest may be an analyte or other protein which it is desired to purify from a starting material. The protein of interest may be an antibody or antibody fragment. The protein (e.g. antibody) could be useful in diagnostic or therapeutic methods. Thus, the method above described may be used in order to ensure that the diagnostic or therapeutic protein is free from contaminating single stranded DNA so that it may be safe to administer. The protein of interest may be a DNA binding protein or other protein which associates with nucleic acid, particularly single stranded DNA in solution. Accordingly, the preparation may be derived from a cell lysate or tissue sample or body fluid and/or may be the product of a nucleic acid amplification or a reverse transcription reaction. The method of the invention can therefore be considered as encompassing a method for refining or enriching a sample comprising double stranded DNA, RNA and/or DNA:RNA duplexes by removing single stranded DNA. Such a method would comprise contacting the sample with an exonuclease as defined above.

In preferred embodiments the sample is the product of a nucleic acid amplification reaction or the product of a reverse transcription reaction and thus the invention provides a method of removing single stranded DNA from the product of a nucleic acid amplification reaction or the product of a reverse transcription reaction, said method comprising the use of an exonuclease as defined above. The method will typically comprise contacting the product of a nucleic acid amplification reaction or the product of a reverse transcription reaction with an exonuclease as defined above.

The exonucleases of the invention are thus used to degrade single stranded DNA present in the product of a nucleic acid amplification reaction or the product of a reverse transcription reaction. In particular, the method involves contacting the product of a nucleic acid amplification reaction or the product of a reverse transcription reaction with an exonuclease of the invention under conditions which permit the digestion of at least a portion of the single stranded DNA present therein and optionally heating the mixture to inactivate said exonuclease. The features of the steps of digestion and inactivation are described above.

The term “nucleic acid amplification reaction” refers to any in vitro means for increasing the number of copies of a target sequence of nucleic acid or its complementary sequence. Preferably, the amplification methods will involve “thermal cycling”, i.e. involving high temperature cycling. Amplification methods include, but are not limited to, PCR and modifications thereto, 3SR, SDA, LAR or LCR and LAMP and modifications thereto. PCR and LCR and their modifications are thermal cycling methods. Methods may result in a linear or exponential increase in the number of copies of the target sequence. “Modifications” encompass, but are not limited to, real-time amplification, quantitative and semi-quantitative amplification, competitive amplification, hot start PCR, and so on. Reverse transcription maybe combined with other nucleic acid amplification reactions as appropriate.

Preferably the nucleic acid amplification method is a method based on the use of oligonucleotide primers as initiators of nucleic acid synthesis, e.g. the PCRs, LAR, SDA, LAMP and NASBA.

The target nucleic acid for the amplification reaction may be DNA or RNA depending on the selected amplification method. For example, for PCR the target is DNA, although when combined with a reverse transcription step the target can be considered to be an RNA sequence. 3SR amplifies RNA target sequences directly.

“Reverse transcription” is a process in which a single strand RNA template is transcribed into a complementary single stranded DNA (cDNA). These nucleic acid strands exist as a duplex until denaturing conditions are applied. The single stranded DNA may also then be used as a template to form double stranded cDNA in a so-called second strand cDNA synthesis step. Some nucleic acid polymerase enzymes are capable of producing the first cDNA strand and synthesising the second strand to form double stranded cDNA and others are specific for just one of the two steps.

A “product of a nucleic acid amplification reaction” is therefore considered to comprise essentially all of the components obtained directly from the final amplification step of the reaction in question. Other components may be added or certain of the components may undergo some modification or processing, but essentially none of the components, or at least none of the nucleic acid components, will have been removed. Preferably the product of a nucleic acid amplification reaction is the direct product of the final amplification step; however, it might also be preferable for the product of a nucleic acid amplification reaction to undergo a treatment to effect the dephosphorylation of any unincorporated NTPs, e.g. a treatment with an alkaline phosphatase, preferably a thermolabile alkaline phosphatase, for instance the heat-labile shrimp alkaline phosphatase (SAP), prior to treatment with the exonucleases of the invention. References to NTPs herein specifically include references to dNTPs unless context dictates otherwise. In other embodiments the dephosphorylation of NTPs may take place after treatment with the exonucleases of the invention, or at the same time. An advantageous recombinant SAP is available from ArcticZymes™ AS.

Accordingly, the above defined methods of removing single stranded DNA from the product of a nucleic acid amplification reaction or the product of a reverse transcription reaction may further comprise using an alkaline phosphatase, preferably a thermolabile alkaline phosphatase, to dephosphorylate any unincorporated NTPs prior to, at the same time as, or after using the exonuclease to remove single stranded DNA, i.e. prior to, at the same time as, or after the step of contacting the product of the amplification or reverse transcription reaction with the exonuclease. In particular, using an alkaline phosphatase in these methods involves contacting the alkaline phosphatase with the product of the amplification/reverse transcription reaction prior to, at the same time as, or after contacting said product with the exonuclease of the invention.

Preferably, any alkaline phosphatase treatment may precede any heat inactivation step. In embodiments where it does not, a further heat inactivation step may be used.

In structural terms, a product of a nucleic acid amplification reaction in accordance with the invention will comprise the template nucleic acid and NTPs, and typically in addition a polymerase and at least one oligonucleotide primer, which may be partially extended. Also typically the bulk of the product will be made up of a suitable nucleic acid amplification buffer, e.g. a buffer as exemplified herein. Preferred products of nucleic acid amplification reactions comprise all of these elements.

In these embodiments the treatments of the invention remove (i.e. degrade), as appropriate, excess non-extended DNA primers, partially extended DNA primers, single stranded DNA template, single stranded DNA amplicons, denatured DNA amplicons and so on.

A “product of a reverse transcription reaction” should be construed in accordance with the definition of a product of a nucleic acid amplification reaction, keeping in mind that a reverse transcription reaction might have only one RNA-dependent nucleic acid polymerisation step and, optionally, may have one or more second strand cDNA synthesis step(s). Preferably the product of a reverse transcription reaction to which the exonuclease of the invention is applied is a product comprising cDNA:RNA duplexes and/or double stranded cDNA.

In a further aspect of the invention there is provided a method of nucleic acid amplification, said method comprising a step subsequent to the final amplification step of using an exonuclease as defined herein to remove single stranded DNA from the product of the nucleic acid amplification reaction. Typically, the step of removing single stranded DNA comprises contacting the product of the nucleic acid amplification reaction, preferably the direct product of the final amplification step, with an exonuclease as defined herein. Said method may further comprise a step, subsequent to the final amplification step, of using an alkaline phosphatase, preferably a thermolabile alkaline phosphatase, to dephosphorylate any unincorporated NTPs in the product of the nucleic acid amplification reaction prior to, at the same time as, or after using the exonuclease to remove single stranded DNA, i.e. prior to, at the same time as, or after the step of contacting the product of the amplification reaction with the exonuclease. In particular, using an alkaline phosphatase in these methods involves contacting the alkaline phosphatase with the product of the amplification reaction prior to, at the same time as, or after contacting said product with the exonuclease of the invention.

In a further aspect of the invention there is provided a method of reverse transcription, said method comprising a step subsequent to the final reverse transcription step, and/or, if present, the final second strand cDNA synthesis step, of using an exonuclease as defined herein to remove single stranded DNA from the product of the reverse transcription reaction. Typically, the step of removing single stranded DNA comprises contacting the product of the reverse transcription reaction, preferably the direct product of the final reverse transcription step, and/or, if present, the direct product of the final second strand cDNA synthesis step, with an exonuclease as defined herein. Preferably such products will be products comprising cDNA:RNA duplexes and/or double stranded cDNA. Said method may further comprise a step, subsequent to the final reverse transcription step, and/or, if present, the final second strand cDNA synthesis step, of using an alkaline phosphatase, preferably a thermolabile alkaline phosphatase, to dephosphorylate any unincorporated NTPs in the product of the reverse transcription reaction prior to, at the same time as, or after using the exonuclease to remove single stranded DNA, i.e. prior to, at the same time as, or after the step of contacting the product of the reverse transcription reaction with the exonuclease. In particular, using an alkaline phosphatase in these methods involves contacting the alkaline phosphatase with the product of the reverse transcription reaction prior to, at the same time as, or after contacting said product with the exonuclease of the invention.

The exonucleases of the invention are thus used to degrade single stranded DNA present in the product of a nucleic acid amplification reaction or the product of a reverse transcription reaction. In particular, the methods involve contacting the product of a nucleic acid amplification reaction or the product of a reverse transcription reaction with an exonuclease of the invention under conditions which permit the digestion of at least a portion of the single stranded DNA present therein and optionally heating the mixture to inactivate said exonuclease. The features of the steps of digestion and inactivation are described above. Preferably, any alkaline phosphatase treatment may precede the heat inactivation step. In embodiments where it does not, a further heat inactivation step may be used.

It is commonplace to combine reverse transcription with one or more nucleic acid amplification reactions. It is also commonplace to combine a plurality of nucleic acid amplification reactions into a single multistage protocol. It will be immediately apparent that each part of these multistage protocols may be considered a method of nucleic acid amplification or method of reverse transcription of the invention in their own right and thus the exonuclease of the invention may be used in accordance with the invention in any or all parts of such multistage protocols.

In a further aspect of the invention there is provided a method of optimising nucleic acid sequence analysis, said method comprising using an exonuclease as defined herein to remove single stranded DNA from the sample to be analysed. The method will typically comprise contacting the sample with an exonuclease as defined herein.

Preferably in this aspect the sample is the product of an amplification reaction or the product of a reverse transcription reaction, e.g. as defined above, but other samples may be used, e.g. those prepared directly from biological materials or which contain biological materials, e.g. microorganisms, body fluids, eukaryotic cells, cultures, tumours and tissues. Preferably the sample will be at least partially purified nucleic acid preparations of the aforementioned samples, e.g. DNA and RNA preparations.

The term “optimisation” encompasses an improvement in the accuracy and/or sensitivity of the sequence analysis of nucleic acids (e.g. the products of nucleic acid amplification or reverse transcription reactions). In accordance with the invention this improvement is, at least in part, a result of the removal of single stranded DNA which may interfere and/or compete with the sequence analysis reactions, reducing their effectiveness against the target template, and/or which are themselves sequenced and so contribute to the background noise in the system, thus making analysis of the outputted signals less sensitive.

In a further aspect of the invention there is provided a method of nucleic acid sequence analysis, said method comprising a step of sample preparation prior to the analysis step(s) of using an exonuclease as defined herein to remove single stranded DNA from the sample to be analysed. The method will typically comprise contacting the sample with an exonuclease as defined herein.

In these latter aspects where the sample is a product of an amplification reaction or the product of a reverse transcription reaction, the sample may also undergo treatment with an alkaline phosphatase, preferably a thermolabile alkaline phosphatase, to dephosphorylate any unincorporated NTPs. Accordingly, the above defined method of nucleic acid sequence analysis may comprise a further sample preparation step of using an alkaline phosphatase to dephosphorylate any incorporated NTPs prior to, at the same time as, or after using the exonuclease to remove single stranded DNA, i.e. prior to, at the same time as, or after the step of contacting the sample to be analysed with the exonuclease. In particular, using an alkaline phosphatase in these methods involves contacting the alkaline phosphatase with the sample to be analysed prior to, at the same time as, or after contacting said product with the exonuclease of the invention.

In these latter aspects the exonucleases of the invention are thus used to degrade single stranded DNA present in the sample to be analysed, e.g. those samples discussed above. In particular, the method involves contacting the sample with an exonuclease of the invention under conditions which permit the digestion of at least a portion of the single stranded DNA present therein and optionally heating the mixture to inactivate said exonuclease. The features of the steps of digestion and inactivation are described above. Preferably, any alkaline phosphatase treatment may precede the heat inactivation step. In embodiments where it does not, a further heat inactivation step may be used.

Preferably in these latter aspects the nucleic acid sequence analysis is a sequencing technique, e.g. the Sanger dideoxynucleotide sequencing method or a “next generation” or “second generation” sequencing approach (for instance, those involving pyrosequencing, reversible terminator sequencing, cleavable probe sequencing by ligation, non-cleavable probe sequencing by ligation, DNA nanoballs, and real-time single molecule sequencing) or an oligonucleotide hybridisation probe based approach in which the presence of a target nucleotide sequence is confirmed by detecting a specific hybridisation event between a probe and its target.

The nucleic acid sequence analysis may provide information useful in the genotyping of an organism, e.g. for classification, identification, quantification, prognostic, diagnostic and/or forensic applications, or useful in the profiling of the transcriptome of a cell or group of cells, e.g. for prognostic, diagnostic and/or research applications. The invention encompasses the use of the exonucleases defined herein in the methods described here for such purposes.

The invention also provides the use of an exonuclease as defined herein to remove single stranded DNA from a sample, and more specifically in the methods described herein.

Where appropriate, the exonucleases of the invention may be isolated from a natural source, e.g. isolated from extracts of the organisms described above, or produced recombinantly in a host cell and isolated and purified therefrom. The exonucleases of the invention may therefore be recombinant enzymes, in particular isolated recombinant enzymes. In certain embodiments the exonuclease is produced by recombinant techniques in a host cell that is not, or not from, an organism which is the same as that in which the exonuclease is found naturally, i.e. a heterologous host cell. Alternatively, a cell-free expression system can be used for production of the exonuclease. These approaches may result in an altered glycosylation pattern.

A method for the isolation and purification of an exonuclease or an enzymatically active fragment thereof as described herein represents a further aspect of the present invention. Thus, in this aspect the invention provides such a method, said method comprising culturing cells in which the exonuclease is expressed and subsequently separating the exonuclease from said cells and/or the media in which said cells have been cultured. Preferably the method comprises expressing said exonuclease in a suitable heterologous host cell (e.g. E. coli, Bacillus and Pichia pastoris), and subsequently separating the exonuclease from said host cells and/or the media in which said cells have been cultured. Expression of said exonuclease can be achieved by incorporating into a suitable host cell an expression vector encoding said nuclease, e.g. an expression vector comprising a nucleic acid molecule encoding any of the amino acid sequences of SEQ ID NOs. 1 to 5 or 11 to 15, for instance a nucleic acid molecule comprising the nucleotide sequence of any of SEQ ID NOs. 6 to 10 or 16 to 20. Host cells comprising these expression vectors and nucleic acid molecules are encompassed by the invention.

The exonuclease enzyme may be separated, or isolated, from the host cells/culture media using any of the purification techniques for protein known in the art and widely described in the literature or any combination thereof. Such techniques may include for example, precipitation, ultrafiltration, dialysis, various chromatographic techniques, e.g. gel filtration, ion-exchange chromatography, affinity chromatography, electrophoresis, centrifugation etc. As discussed above, the exonuclease of the invention may be modified to carry amino acid motifs or other protein or non-protein tags, e.g. polyhistidine tags, to assist in isolation, solubilisation and/or purification or identification. Examples of polyhistidine tagged exonucleases of the invention are recited in SEQ ID Nos 11 to 15.

Likewise an extract of host cells may also be prepared using techniques well known in the art, e.g. homogenisation, freeze-thawing etc. and from this extract the exonuclease of the invention can be purified.

Freezer storage of the exonuclease of the invention may be conveniently achieved with a storage buffer of 5 mM Tris/HCl, pH 7.5 (at 25° C.), 250 mM NaCl, 5 mM MgCl₂, 0.25 mM EDTA, 50% glycerol, although other buffers may be used.

The present invention also provides kits which comprise at least an exonuclease according to the invention or a nucleic acid encoding an exonuclease according to the invention. The kits may also contain some or all of the necessary reagents, buffers, enzymes etc. to carry out nucleic acid amplification and/or reverse transcription and/or sequence analysis reactions. More particularly, the kits may contain nucleotide triphosphates (including dNTPαS for SDA), oligonucleotide primers, oligonucleotide probes, reverse transcriptases, DNA polymerases, preferably a thermostable polymerase such as Taq polymerase or Bst polymerase (and hot-start versions thereof) or, in the case of LAR, a DNA ligase (preferably a thermostable DNA ligase such as Ampligase® or that disclosed in U.S. Pat. No. 6,280,998 which is isolated from Pyrococcus furiosus) an alkaline phosphatase, preferably a thermolabile alkaline phosphatase (e.g. SAP), or a restriction enzyme (preferably a thermostable restriction enzyme such as BsoB1). Kits comprising an exonuclease of the invention and an alkaline phosphatase, preferably a thermolabile alkaline phosphatase (e.g. SAP), e.g. any of those disclosed herein are of note.

The present invention also provides compositions comprising an exonuclease of the invention and one or more of the necessary reagents to carry out nucleic acid amplification and/or reverse transcription and/or sequence analysis reactions and methods, e.g. those components described above. Typically such compositions will be aqueous and buffered with a standard buffer such as Tris, HEPES, etc. The present invention also provides compositions comprising an exonuclease of the invention in a buffer.

BRIEF DESCRIPTION OF THE FIGURES

The invention will now be described by way of non-limiting Examples with reference to the following figures in which:

FIG. 1 shows the amino acid and nucleotide sequences of Shewanella sp. exonuclease (SEQ ID No 1 and SEQ ID NO:6, respectively).

FIG. 2 shows the amino acid and nucleotide sequences of Halomonas sp. exonuclease (SEQ ID No 2 and SEQ ID NO:7, respectively).

FIG. 3 shows the amino acid and nucleotide sequences of Vibrio wodanis exonuclease (SEQ ID No 3 and SEQ ID NO:8, respectively).

FIG. 4 shows the amino acid and nucleotide sequences of Psychromonas sp. exonuclease (SEQ ID No 4 and SEQ ID NO:9, respectively).

FIG. 5 shows the amino acid and nucleotide sequences of Moritella viscosa (SEQ ID NO:5 and SEQ ID NO:10, respectively).

FIG. 6 shows an alignment of SEQ ID Nos:1-5 with the amino acid sequence of E. coli exonuclease I (SEQ ID NO:22) generated with ClustalW Multiple alignment tool. The consensus sequence is shown at the bottom. *, identical residues in all sequences; highly conserved residues among the sequences; weakly conserved residues among the sequences.

FIG. 7 shows the amino acid and nucleotide sequences of a His-tagged version of Shewanella exonuclease (SEQ ID No 11 and SEQ ID NO:16, respectively).

FIG. 8 shows the amino acid and nucleotide sequences of a His-tagged version of Halomonas exonuclease (SEQ ID NO:12 and SEQ ID No. 17, respectively).

FIG. 9 shows the amino acid and nucleotide sequences of a His-tagged version of Vibrio wodanis exonuclease (SEQ ID NO:13 and SEQ ID NO:18, respectively).

FIG. 10 shows the amino acid and nucleotide sequences of a His-tagged version of Psychromonas exonuclease (SEQ ID NO:14 and SEQ ID NO:19, respectively).

FIG. 11 shows the amino acid and nucleotide sequences of a His-tagged version of Moritella viscosa exonuclease (SEQ ID NO:15 and SEQ ID NO:20, respectively).

FIGS. 12A-12D shows images of a number of polyacrylamide gels on which the products of a variety of reactions between a single stranded DNA oligonucleotide and a variety of heat-labile (HL) exonucleases have been separated, thus indicating activity of the enzyme against single stranded DNA. Buffer conditions as described in Example 2. (−) Cont—negative control. FIG. 12A: HL-ExoI (Ha) activity at different temperatures (20 to 37° C.) at different time intervals (1 to 10 minutes). Different dilution factors depending on incubation time (4× for 1 to 5 minutes and 10× for 10 minutes). FIG. 12B: HL-ExoI (Ps) activity at different temperatures (20 to 37° C.) at different time intervals (1 to 10 minutes). Different dilution factors depending on incubation time (3× for 1 minute, 8× for 5 minutes and 12× for 10 minutes). FIG. 12C: HL-ExoI (Sh) activity at different temperatures (20 to 37° C.) at different time intervals (1 to 10 minutes). Different dilution (4× for 1 minute and 5 minutes and 10× for 10 minutes). FIG. 12D: HL-ExoI (Mv) activity at different temperatures (20 to 37° C.) at different time intervals (1 to 10 minutes). Different dilution factors depending on incubation time (1× for 1 minute and 5 minutes and 2× for 10 minutes).

FIG. 13 shows an image of a polyacrylamide gel on which the products of a variety of reactions between a single stranded DNA oligonucleotide and a variety of heat treated exonucleases have been separated, thus indicating activity of the enzymes against single stranded DNA. Buffer conditions as described in Example 3. (−) Cont—negative control. Four commercially available E. coli ExoI enzymes (ExoI A-D), one commercially available enzymatic PCR clean-up kit and HL-ExoI (Sh) were compared in terms of ease of thermal inactivation. Samples were incubated for 1 min at 80° C. before substrate addition and residual activity incubation.

FIGS. 14A-14C shows representative sequencing results from the sequencing reactions of Example 4. FIG. 14A: reaction based on GoTaq PCR buffer—ExoSAP-IT corresponds the 30 minutes protocol described in Example 4 and HL-ExoI (Sh)/SAP corresponds to the 5 minutes protocol described in Example 4. FIG. 14B: as FIG. 14A, respectively, although TEMPase Extra PCR buffer used in place of GoTaq. FIG. 14C: as FIG. 14A although ExoSAP-IT and HL-ExoI (Sh)/SAP corresponds to the 5 min protocol described in Example 4.

FIGS. 15A-15E shows images of a number of polyacrylamide gels on which the products of a variety of reactions between single stranded DNA oligonucleotides and HL-ExoI of the invention have been separated, thus indicating the residual activity of the enzyme following heat treatment against single stranded DNA. Buffer conditions as descried in Example 5. 15A: HL-ExoI (Ha); 15B: HL-ExoI (Sh); 15C: HL-ExoI (Ps); 15D: HL-ExoI (Mv); 15E: HL-ExoI (Vw). (−) Cont—negative control. Samples were incubated for 5 minutes, 10 minutes or 15 minutes at different temperatures (40° C.-60° C.) prior to substrate addition and residual activity incubation, which was performed at 30° C. for 30 minutes, and then for 2 minutes at 95° C.

FIGS. 16A-16E shows images of a number of polyacrylamide gels on which the products of a variety of reactions between single stranded DNA oligonucleotides and HL-ExoI of the invention have been separated, thus indicating the activities of the test ExoI against single stranded DNA at increasing dilution. Buffer conditions as descried in Example 6. 16A: HL-ExoI (Ha); 16B: HL-ExoI (Sh); 16C: HL-ExoI (Ps); 16D: HL-ExoI (Mv); 16E: HL-ExoI (Vw). (−) Cont—negative control. 100%—undiluted enzyme, 10% —enzyme diluted 10 times, 1%—enzyme diluted 100 times, 0.1%—enzyme diluted 1000 times, 0.01%—enzyme diluted 10,000 times. Samples were incubated for 30 minutes at 30° C., and then for 2 minutes at 95° C.

FIG. 17 shows the 3′ to 5′ directionality of HL-ExoI (Ha, Ps, Sh, Vw, Mv) as well as for E. coli ExoI. Assay conditions are described in Example 11. When FAM was labeled at the 5′ end, a ladder of partial faint and intense intermediate product bands were seen indicating the ExoI degrading the substrate from the 3′ end. When the oligo was FAM-labeled at the 3′ end, the fluorophore was immediately cut off, generating only the 3′FAM monomer. (−)Cont—negative control. ExoI—E. coli ExoI.

FIG. 18 shows the crystal structure of HL-ExoI (Mv) in complex with ssDNA (dT13) at a resolution of 2.5 Å. Experimental set-up is described in Example 12. Active site residues Asp23, Glu25 and Asp194 are indicated as sticks. In the three-dimensional structure of the HL-ExoI (Mv) the 3′-end of the dT13 is clearly located in the active site of the enzyme.

FIG. 19 shows a polyacrylamide gel on which the activity and inactivation of HL-ExoI (Ps, Sh, Vw, Mv) and E. coli ExoI were compared. Buffer conditions and reaction set up are as described in Example 12. All enzymes were tested for activity at 30° C. for 15 minutes as well as residual activity under the same time and temperature conditions following incubation at 80° C. for 1 minute. To mimic a post-PCR clean-up assay, reaction was performed in a post-PCR buffer.

FIG. 20 shows a polyacrylamide gel on which the activity and inactivation of HL-ExoI (Sh) and E. coli ExoI were compared. Buffer conditions and reaction set up are as described in Example 12. All enzymes were incubated at 80° C. for 1, 5, 10 or 20 minutes before substrate addition and incubation at 30° C. for 15 minutes. To mimic a post-PCR clean-up assay, reaction was performed in a post-PCR buffer. No residual activity in HL-ExoI (Sh) was detected after 1 min incubation at 80° C., while substantial residual activity was observed with two commercial E. coli ExoI after 5 minutes, and even after 20 minutes incubation at 80° C. in the case of ExoI A.

FIGS. 21A-21F shows representative sequencing results from the sequencing reactions of Example 14 (A: Negative control; B: ExoSAP-IT; C: HL-ExoI (Sh)/SAP; D: HL-ExoI (Ps)/SAP; E: HL-ExoI (Mw)/SAP; F: HL-ExoI (Vw)/SAP. All images show the resulting chromatograms following addition of excess reverse primer prior to PCR clean-up. All sequencing results were based upon the GoTaq PCR buffer. ExoSAP-IT corresponds to the 30 minutes protocol, while HL-ExoI/SAP corresponds to the 5 minutes protocol, both described in Example 14.

EXAMPLES Example 1—Cloning, Recombinant Expression and Partial Purification of Exonucleases

The sbcB exodeoxyribonuclease I (exoI) gene from Moritella viscosa (HL-ExoI (Mv)), Vibrio wodanis (HL-ExoI (Vw)), Halomonas (HL-ExoI (Ha)), Psychromonas (HL-ExoI (Ps)) and Shewanella (HL-ExoI (Sh)) gDNA was cloned by overlap extension PCR cloning (Bryksin and Matsumura, 2010, Biotechniques, 48 (6), 463-465) with a C-terminal His-tag by inserting the cloned gene into the pTrc99A expression vector for expression in E. coli TOP10. The primers used are listed in Table 1. Genetic source material was obtained from bacteria isolated from Norwegian offshore waters.

TABLE 1 Primers used for cloning of ExoI genes. Primer SEQ ID name Sequence No Mv forward GTGAGCGGATAACAATTTCACACAGGAAACAGACCATGG 23 ATAACAATTCGAACAAAACAGCAACAG Mv reverse GCTGAAAATCTTCTCTCATCCGCCAAAACAGCCtcagtgatggt 24 gatggtgatg-gcctgcagaTGCGCCAATTATTTTTTGACCATAAAGG Vw forward GTGAGCGGATAACAATTTCACACAGGAAACAGACCATGCC 25 GCAGGATAACGCACCAAG Vw reverse GCTGAAAATCTTCTCTCATCCGCCAAAACAGCCtcagtgatggtga 26 tggtgatggcctgcagaTGATACTAACTGTTGTACGTAATTATAAACG GCGC Ha forward GTGAGCGGATAACAATTTCACACAGGAAACAGACCATGGCA 27 TCACCCAATGCTGCC Ha reverse GCTGAAAATCTTCTCTCATCCGCCAAAACAGCCtcagtgatggtga 28 tggtgatggcctgcagaGGCATCAAATGCCTGGGCCG Ps forward GTGAGCGGATAACAATTTCACACAGGAAACAGACCATGAAT 29 CAAGAATCCCCAAGCCTTCTTTGG Ps reverse GCTGAAAATCTTCTCTCATCCGCCAAAACAGCCtcagtgatggtga 30 tggtgatggcctgcagaTGTATTCCCTGTCAAAAACTCTAAGTAATGT CC Sh forward GTGAGCGGATAACAATTTCACACAGGAAACAGACCATGAAC 31 AACACTAAGAAACAGCCAACTTTATTTTGG Sh reverse GCTGAAAATCTTCTCTCATCCGCCAAAACAGCCtcagtgatggtga 32 tggtgatggcctgcagaAAGATTTCTAAGATAATGACACAAAGCCTGT AA Bold letters, pTrc99A specific sequence; upper case letters, gene specific sequence; lower case letters, spacer and tag sequence.

The pTrc99A-exoI vectors were transformed into E. coli TOP10 following the protocol for Z-competent cells (Zymo Research, U.S.A.). The cells were grown in baffled shake flasks in Terrific Broth (TB) medium; approximately 1.5% overnight precultures were transferred to 250 ml TB medium containing 100 μg/ml Ampicillin in 1000 ml growth flasks and incubated at 37° C., 200 rpm, until the OD₆₀₀ reached 0.4-0.6. The temperature was decreased to 15° C. and the cells were incubated for 30 minutes before they were induced for 4 hours with 0.5 mM IPTG. The cells were harvested by centrifugation and the cell pellets frozen at −20° C.

Cell pellets from the 250 ml cultures were thawed on ice, 40 ml lysis buffer (50 mM Tris-HCl (pH 7.5 at 25° C.), 5 mM imidazole, 1 M NaCl, 0.1% Triton X-100, 10% glycerol, 10 mM MgCl₂) was added and the mixture was sonicated in an ice water bath for 10 min (25% amplitude, 0.1 sec on, 0.2 sec off) using a Branson Sonifier. The lysate was centrifuged in a 50 ml tube at 25,000 g for 20 minutes and the supernatant filtered through a 0.45 μM filter. The filtered lysate was diluted with 50 ml of lysis buffer to a total volume of 90 ml. All purification steps were performed with ice cold buffers and a column cooled on ice. The 90 ml lysate was applied to a HisTrap HP 1 ml column equilibrated in lysis buffer using a flow of 1 ml/min. The column was washed with 5 column volumes (CV) of lysis buffer and 10 CV of buffer A2 (50 mM Tris-HCl, (pH 7.5 at 25° C.), 5 mM imidazole, 500 mM NaCl). The protein was then eluted with 20 CV of a 0-30% gradient of buffer B (50 mM Tris-HCl, (pH 7.5 at 25° C.), 500 mM Imidazole, 500 mM NaCl) to buffer A2 in 1 ml fractions. Fractions containing ExoI activity were pooled and either dialysed against 10 mM Tris-HCl (pH 7.5 at 25° C.), 500 mM NaCl and 0.5 mM EDTA, or 10 mM Tris-HCl (pH 7.5 at 25° C.), 500 mM NaCl, 10 mM MgCl₂ and 0.5 mM EDTA, and then diluted 1:1 with 100% glycerol and stored at −20° C.

Two more exonuclease I sequences (not disclosed) have been examined, but failed to express and isolate actively.

Example 2—Activity Profiling of Exonucleases: Optimum Temperature for Catalytic Activity

A temperature profile was created to better characterise the different recombinant HL-ExoI from Example 1. The different HL-ExoI were diluted to concentrations enabling differentiation between substrate degradations on the gel. Due to the different dilution factors, samples could not directly be compared to each other, but relative temperature-dependent differences in activity could be estimated. Samples were incubated for various time intervals to determine if the enzyme could sustain a set temperature for longer periods of time.

Detailed Method

HL-ExoI was diluted (10 mM Tris-HCl pH 7.5 at 25° C., 5 mM MgCl₂) to what was thought to give the best differentiation between samples. To mimic the PCR clean-up protocol, post-PCR solutions were used as reaction buffers. Robustness was achieved by running the experiment in parallel using post-PCR solutions based on two different PCR buffers; GoTaq (Promega) or TEMPase Key (VWR). As substrate, 5 pmol of a 5′ FAM labeled oligonucleotide (GCTAACTACCACCTGATTAC; SEQ ID No 21) was added to each reaction. Following addition of the ExoI, the total volume for each sample was 7 μl. Samples were incubated in a thermocycler for 1 minute, 5 minutes or 10 minutes at 20° C., 25° C., 30° C. or 37° C. followed by 5 minutes at 80° C. A TBE-Urea Sample Buffer (Bio-Rad) was added and samples were applied to a casted 20% Acrylamide/7M Urea gel and run at 180 V for approximately 45 minutes. All reagents were kept on ice during the full protocol unless specified otherwise and workflow was performed on cooling blocks.

Results

Results are shown in FIG. 12. In general, all HL-ExoI performed similarly in the two buffers used to prepare the post-PCR samples, showing that the observed effects were not specific to the PCR-buffer composition.

HL-ExoI (Ha) showed an overall increasing activity up to 37° C. and could withstand this temperature for at least 10 minutes (FIG. 12A). Good overall activity at lower temperatures was also seen.

HL-ExoI (Ps) showed an overall best activity at 30° C., with loss of activity when using 37° C. as incubation temperature (FIG. 12B). Good overall activity at lower temperatures was also seen.

HL-ExoI (Sh) showed an overall best activity at 37° C., and could withstand this temperature for at least 10 minutes (FIG. 12C). Good overall activity at all temperatures was also seen.

HL-ExoI (Mv) showed an overall best activity at 30° C., while incubation at 37° C. resulted in loss of activity (FIG. 12D). Good activity at lower temperatures was also seen.

Using PAGE for activity profiling appeared to give good estimates as to how the HL ExoI behaved depending on temperature over time.

Example 3—Activity Profiling of Exonucleases: Inactivation Temperatures for Catalytic Activity

In this Example, the thermal inactivation characteristics of HL-ExoI (Sh) were compared to various commercially available E. coli ExoI.

The thermal inactivation characteristics of four different commercially available E. coli ExoI and one commercially available enzymatic PCR clean-up kit were compared to HL-ExoI (Sh). To ensure that any observed effects were irrespective of the choice of reaction buffer, two different post-PCR solutions were used as a reaction buffer (TEMPase Key, TEMPase Extra, VWR). All reactions received about 10 U of ExoI to enable comparison between the ExoI activities and ease of thermal inactivation. The exonuclease activity of HL-ExoI (Sh) was calculated as described in Example 8. For the commercial ExoIs, exonuclease activity was taken as that stated by the manufacturers. Final volume for each reaction was 7 μl. Samples were incubated at 80° C. for 1 minute before cooling and addition of 5 pmol of a 5′ FAM labeled oligonucleotide (GCTAACTACCACCTGATTAC; SEQ ID No 21). Samples were further incubated at 37° C. for 15 minutes. Following incubation, a TBE-Urea Sample Buffer (Bio-Rad) was added and samples were applied to a casted 20% acrylamide/7 M urea gel and run at 180 V for approx. 45 minutes. All reagents were kept on ice during the full protocol and workflow was performed on cooling blocks unless specified otherwise.

Results are shown in FIG. 13. All of the E. coli ExoI had adequate activity following 1 minute incubation at 80° C. to completely degrade all of the substrate. HL-ExoI (Sh) was the only ExoI that was completely inactivated, showing no signs of residual activity.

Example 4—Demonstration of Utility of HL-ExoI (Sh) in a Rapid PCR Clean-Up Prior to Nucleic Acid Sequencing

Experiments were set up to verify that HL-ExoI (Sh) could perform satisfactorily in a rapid PCR clean-up scenario. For comparison and positive control, parallel samples were treated with a leading brand of enzymatic PCR clean-up reagent ExoSAP-IT (Affymetrix).

To verify that HL-ExoI (Sh) enabled a 5 minute enzymatic PCR clean-up protocol, an experiment was designed to stress-test the protocol limitations. Thus, to all post-PCR solutions under test there were added excess primers or dNTPs following the PCR. If left unremoved prior to the sequencing reaction, residual primers would result in sequence reactions in the opposite direction and thereby strongly compromise the length and quality of the reaction, and dNTPs would result in ddNTP:dNTP ratios which would fail to yield high quality sequences. Robustness was achieved by using different PCR reagents and amplicons.

Following PCR, 10 pmol primers or 40 nmol dNTPs were added to the post-PCR solutions.

Samples to be treated with ExoSAP-IT were handled according to manufacturer protocol. Following addition of either reverse primer or dNTP to the PCR solution, samples received 2 μl of the clean-up reagent, giving a final volume of 7 μl. Samples were incubated 15 minutes at 37° C. followed by 15 minutes at 80° C. Samples were set up using two different PCR buffers and all samples were set up as triplicates.

Samples to be treated with HL-ExoI (Sh) received the same amount of added primers and dNTPs as above, before addition of 1 μl of HL-ExoI (Sh) (10 U/μl, as calculated in Example 8) and 1 μl of SAP (2 U/μl). As with the positive controls, the final volume of the samples were 7 μl. Samples were incubated 4 minutes at 37° C. followed by 1 minute at 80° C. Samples were set up using two different PCR buffers and all samples were set up as triplicates.

To evaluate how ExoSAP-IT would perform given a protocol identical to HL-ExoI, samples were spiked with reverse primers before the addition of 2 μl of the ExoSAP-IT PCR clean-up reagent. Samples were set up as duplicates.

As negative controls, samples received reverse primers or dNTPs, but instead of enzymatic clean-up solutions, samples received 2 μl water. Samples were set up as duplicates.

TABLE 2 provides an overview of the above described experimental set up. Reagent Volume Post-PCR solution (TEMPase Extra, VWR or GoTaq, Promega) 4 μl Spike dNTP (10 mM ACGT each) or Reverse primer (10 μM) 1 μl PCR clean-up ExoSAP-IT (commercially available PCR clean-up solution) or 2 μl HL-Exol & SAP or dH₂O Total 7 μl

TABLE 3 provides an overview of the PCR clean-up incubation Protocol Incubation Total time ExoSAP-IT 15 minutes at 37° C. → 15 minutes at 80° C. 15 minutes 4 minutes at 37° C. → 1 minute at 80° C. 5 minutes HL-Exol + 4 minutes at 37° C. → 1 minute at 80° C. 5 minutes SAP Negative 15 minutes at 37° C. → 15 minutes at 80° C. 15 minutes control

Following PCR clean-up treatment, samples were immediately cooled on ice. A prepared sequencing reaction mastermix was aliquoted into separate tubes. A total of 2.5 μl of each treated/untreated solution was added to each tube to be used as a template in the subsequent sequencing reaction. Samples were immediately transferred to a thermocycler and the sequencing program was initiated.

Reagent Volume BigDye v3.1 (LifeTech) 1 μl 5X Sequencing Buffer (LifeTech) 4 μl Sequencing primer (10 μM) 0.32 μl Template (treated/untreated PCR product with spike) 2.5 μl dH₂O 12.18 μl Total 20 μl

Cycling temperature Time 96° C. 5 min 96° C. 10 sec 50° C. 5 sec {close oversize bracket} 25 cycles 60° C. 4 min  4° C. Hold

Sequences were delivered to the DNA Sequencing core Facility at University of Tromsø for purification and sequencing using an Applied Biosystems 3130xl Genetic Analyzer. Results were analyzed using the Sequence Scanner Software v1.0 (LifeTech).

Selected results are shown in FIG. 14. Sequences spiked with dNTP yielded an overall good sequence length and quality, and no difference between samples treated with ExoSAP-IT or HL-ExoI/SAP could be detected (results not shown). The sequence plots of FIG. 14A are examples of the results from sequences spiked with reverse primers in the GoTaq PCR buffer. It was evident from the negative controls that lack of functional PCR clean-up strongly compromised the length and quality of the sequence. Samples treated with either ExoSAP-IT (30 minutes protocol) or HL-ExoI/SAP (5 minutes protocol) showed very good sequence quality. These images were representative for all replicates.

The sequence plots of FIG. 14B are examples of the results from sequences spiked with reverse primers in the TEMPase Extra PCR buffer. Samples with added reverse primers showed very good sequence length and quality upon treatment with either of the PCR clean-up solutions. There were no significant differences between the ExoSAP-IT-treated samples (30 minutes protocol) or the HL-ExoI/SAP-treated samples (5 minutes protocol). Lack of PCR clean-up treatment resulted in shorter sequences of lower quality. These images were representative for all replicates.

On the other hand FIG. 14C illustrates how the ExoSAP-IT clean-up solution performed when having to perform the same 5 minutes protocol as HL-ExoI/SAP-protocol. Evident from the Figure is that treatment with ExoSAP-IT did not result in sequences of high quality. This is likely due to a combination of insufficient degradation of added primers as well as residual ExoI activity degrading sequencing primers. It is likely that using a reaction set-up at room temperature would further compromise these results due to the residual ExoI activity degrading sequencing primers. Samples treated with HL-ExoI/SAP showed overall excellent sequence length and quality.

Example 5—Inactivation Experiments to Determine Minimum Inactivation Time and Temperature for Certain Heat-Labile Exonucleases of the Invention

Minimum inactivation temperature and time was determined for each HL-ExoI under test under the given assay conditions. This was achieved by incubating HL-ExoI at different temperatures for different time intervals. Following heat-treatment, 5′ labeled single stranded DNA was added and degree of substrate degradation was visualized (FIG. 15). The amount of substrate degradation was compared to the results from FIG. 16, and residual activity following heat treatment was estimated.

Undiluted HL-ExoI was added to Reaction Buffer (10 mM Tris-HCl, pH 8.5 at 25° C., 50 mM KCl, 5 mM MgCl₂), giving a final volume of 9 μl. Samples were incubated for 5 minutes, 10 minutes or 15 minutes at 40° C., 45° C., 50° C. or 55° C. or for 5 minutes or 10 minutes at 60° C. Following cooling of samples, 5 pmol of a 5′FAM labeled oligonucleotide (GCTAACTACCACCTGATTAC; SEQ ID No 21) was added. Samples were further incubated at 30° C. for 30 minutes and then for 2 minutes at 95° C. A TBE-Urea Sample Buffer (Bio-Rad) was added and samples were applied to a precast 20% acrylamide/7 M urea gel and run at 180 V for approximately 45 minutes. All reagents were kept on ice during the full protocol and workflow was performed on cooling blocks unless specified otherwise. PAGE results were imaged using the Molecular Imager PharosFX system (Bio-Rad).

Results are shown in FIG. 15 and indicate that various degrees of substrate degradation are observed depending on temperature and time-interval for heat-incubation. Overall, none of the HL-ExoI showed any signs of substrate degradation following incubation at 55° C. for 10 minutes or more. Incubations of all of the HL-ExoI at 55° C. for 5 minutes or 50° C. for 10 minutes showed essentially no, or at most between 1 and 10%, substrate degradation.

Example 6—Determination of Sensitivity Threshold for Inactivation Experiments by Measuring Degree of Substrate Degradation for a Dilution Series of Exonucleases

In order to determine the minimum inactivation temperature for the various HL-ExoI of the previous Example, the sensitivity threshold for the inactivation assay of Example 5 was determined. A semi-quantitative assay was prepared using serial dilutions of the exonucleases and estimating the degree of substrate degradation for each dilution. For comparative measurements, the same assay conditions and reaction set-up as used in the inactivation assays were also applied here.

Each HL-ExoI under test was diluted 1, 10, 100, 200, 1000 and 10,000 times, corresponding to 100%, 10%, 1%, 0.5%, 0.1% and 0.01% activities. The same buffer as used as Reaction Buffer was also used as Dilution Buffer (10 mM Tris-HCl, pH 8.5 at 25° C., 50 mM KCl, 5 mM MgCl₂). Reaction Buffer and 5 pmol of FAM-labeled substrate (GCTAACTACCACCTGATTAC; SEQ ID No 21) were premixed prior to HL-ExoI addition. Total volume of reaction was 10 μl. Samples were incubated for 30 minutes at 30° C., followed by 2 minutes at 95° C. A TBE-Urea Sample Buffer (Bio-Rad) was added and samples were applied to a precast 20% acrylamide/7M urea gel and run at 180 V for approx. 45 minutes. All reagents were kept on ice during the full protocol and workflow was performed on cooling blocks unless specified otherwise. PAGE results were imaged using the Molecular Imager PharosFX system (Bio-Rad).

Results are shown in FIG. 16 and indicate that activity of the exonucleases could be detected to approximately 1% activity in the given assay conditions.

Example 7—Assay for Determining Double-stranded and Single-stranded Exonuclease Activity

Exonuclease activity is measured by incubating the test exonuclease enzyme with a short 5′-FAM-DNA-TAMRA-3′ labelled single or double stranded substrate of approximately 20 nucleotides. If the exonuclease is able to degrade the substrate this will start immediately and the fluorophore will be released. The activity can be followed over time since the released fluorophore can re-emit light upon light excitation.

Specifically, the assay mix consists of 1 μl 10 μM ssDNA/dsDNA, 10 μl 5×TDB (250 mM Tris-HCl, pH 8.5 at 25° C., 5 mM DTT, 1 mg/ml BSA, 10% Glycerol) and 29 μl MiliQ H₂O. 40 μl assay mix is transferred to the wells of a black flat bottom 96-well plate and 5 μl MiliQ H₂O or dilution buffer (as negative control) and enzyme samples are added to the wells. The reactions are initiated by adding 5 μl 50 mM MgCl₂ using a multichannel pipette, making the final volume of the reaction 50 μl. Fluorescence is measured immediately (excitation 485 nm and emission at 520 nm) and then at appropriate time intervals, include a shaking step, and the reactions are allowed to proceed for 15 minutes. An increase in fluorescence indicates degradation of the substrate is taking place.

Example 8—Assay for Quantifying Single-stranded Exonuclease Activity

Single strand DNA exonuclease activity was measured by incubating the enzyme with a denatured ³H-dATP incorporated PCR product. If the exonuclease is able to degrade the substrate the exonuclease will release acid soluble nucleotides that can be detected in a scintillation counter. Excess high molecular weight substrate DNA is precipitated with trichloroacetic acid (TCA). In this assay, one Unit (1 U) is defined as the amount of enzyme that will catalyse the release of 10 nmol acid-soluble nucleotides in a final volume of 20 μl in 30 minutes at 30° C.

Specifically, the assay mix consisted of 4 μl 5× Exonuclease buffer (250 mM Tris-HCl, pH 7.5 at 25° C., 50 mM MgCl₂, 5 mM DTT), 5 μl denatured substrate (denatured by incubation for 3 minutes at 100° C. and immediate transfer to an ice water bath for 3 minutes) and 6 μl MiliQ H₂O. 15 μl was transferred to 1.5 ml microcentrifuge tubes on ice. The enzyme under test was diluted when necessary and 5 μl each of enzyme sample, control and blank was added to the assay mix and mixed by pipetting up and down. The samples were incubated in a water bath at 30° C. for 10 minutes. After incubation the reactions were placed on ice and 20 μl ice cold calf thymus DNA (1 mg/ml) and 250 μl ice cold 10% (w/v) TCA were added immediately. The samples were then incubated on ice for 15 minutes and centrifuged at 4° C. for 10 minutes at 13,000 rpm. The supernatants, 200 μl, were transferred to a 24-well plate and 0.8 ml Ultima Gold XR Scintillation fluid was then added. The wells were sealed with sealing tape and the samples were mixed thoroughly by shaking. The samples were counted in a MicroBeta² Plate Counter for 5 minutes.

Example 9—Assay for Quantifying Double-stranded Exonuclease Activity

Double strand DNA exonuclease activity is measured in the same way as Example 8 with the exception that the PCR substrate is not denatured prior to incubation with the enzyme.

Example 10—Comparison of the Double Stranded and Single Stranded Exonuclease Activities of the Heat Labile Exonucleases of the Invention

Single strand DNA exonuclease activity was measured by incubating the test enzyme with a denatured ³H-dATP incorporated PCR product. Double strand DNA exonuclease activity was similarly measured with the exception that the enzyme was incubated with a non-denatured ³H-dATP incorporated PCR product. If an exonuclease is able to degrade the substrate the exonuclease will release acid soluble nucleotides that can be detected in a scintillation counter. Excess high molecular weight substrate DNA is precipitated with trichloroacetic acid (TCA). In this assay, one Unit (1 U) is defined as the amount of enzyme that will catalyse the release of 10 nmol acid-soluble nucleotides in a final volume of 20 μl in 30 minutes at 30° C. Specifically, the assay mix consisted of 4 μl 5× buffer (50 mM Tris-HCl, pH 8.5 at 25° C., 250 mM KCl and 25 mM MgCl₂), 5 μl substrate (to obtain ssDNA the dsDNA substrate was denatured by incubation for 3 minutes at 100° C. and immediate transferred to an ice water bath for 3 minutes) and 3 μl MiliQ H₂O. The assay mix, 12 μl, was transferred to 1.5 ml microcentrifuge tubes on ice. The enzyme under test was diluted and 8 μl of enzyme sample was added to the assay mix and mixed by pipetting up and down. Control and blank were also included in the set-up. The samples were incubated in a water bath at 30° C. for 10 minutes. After incubation the reactions were placed on ice and 20 μl ice cold calf thymus DNA (1 mg/ml) and 250 μl ice cold 10% (w/v) TCA were added immediately. The samples were then incubated on ice for 15 minutes and centrifuged at 4° C. for 10 minutes at 13,000 rpm. The supernatants, 200 μl, were transferred to a 24-well plate and added 0.8 ml Ultima Gold XR Scintillation fluid. The wells were sealed with sealing tape and the samples mixed thoroughly by shaking. The samples were counted in a MicroBeta² Plate Counter for 5 minutes.

The results are summarized in Table 4 and shows that the HL-ExoI of the invention display very little activity against double stranded DNA compared to the activity against single stranded DNA. It is however difficult to conclude if the low amounts of activity against double stranded DNA are related to intrinsic properties of the enzymes or is caused merely by contaminations of the enzymes. The two commercial ExoI (ExoI A and ExoI B) tested also displayed very low activity against double stranded DNA.

TABLE 4 Activity of HL-Exol and commercial Exoi against ssDNA and dsDNA. Relative activity Activity Activity against dsDNA against ssDNA against dsDNA and ssDNA (U/μl) (U/μl) (%) HL-Exol Ha 50.9 0.05 0.10 HL-Exol Ps 11.2 0.02 0.18 HL-Exol Sh 65.1 0.01 0.02 HL-Exol Vw 21.0 0.01 0.05 HL-Exol Mv 16.7 0.01 0.06 Exol A 16.7 0.01 0.06 Exol B 41.2 0.01 0.02

Example 11—Activity Profiling of Exonucleases: Directionality Determination Using Urea Polyacrylamide Gel

This experiment was performed in order to verify that the HL-ExoI of the invention exhibited 3′ to 5′ exonuclease activity and no substantial 5′ to 3′ exonuclease activity. The substrate specificity was analysed using either a 5′ FAM or a 3′ FAM-labeled oligonucleotide (GCTAACTACCACCTGATTAC; SEQ ID No 21) on a polyacrylamide gel.

Each HL-ExoI under test was diluted to a final concentration of about 0.1 U/μl. Two master mixes were prepared, one for the 5′ FAM-labeled oligo and one for the 3′ FAM-labeled oligo. The master mix contained Reaction Buffer (10 mM Tris pH 8.5, 50 mM KCl, 5 mM MgCl₂) and the respective FAM-labeled substrate, giving a final amount of 0.25 pmol substrate per reaction. Both a negative control, containing dilution buffer instead of enzyme solution, and a positive control, using Exo I from E. coli, were included. The enzymes were pipetted into the pre-cooled reaction tubes and then the master mix with the reaction buffer and substrate was added. All reactions consisted of a final volume of 10 μl and were incubated at 30° C. for 1 h. Reactions were stopped by adding 2.5 μl of sample loading buffer (95% formamide, 10 mM EDTA, Xylene) and incubated at 95° C. for 5 min. For analysis, 6 μl of the samples were loaded onto a 12% acrylamide/7 M urea gel. The gel was run at 50 W for 1 h 45 minutes. During the set-up of the reactions all reagents and samples were kept on ice.

Results are shown in FIG. 17. Clear differences between the 5′ FAM-labeled and 3′ FAM-labeled substrate are observed, indicating 3′ to 5′ directionality of the different HL-ExoI as well as for the E. coli ExoI control. When FAM was labeled at the 5′ end, a ladder of partial faint and intense intermediate product bands were seen indicating the ExoI degrading the substrate from the 3′ end. When the oligo was FAM-labeled at the 3′ end, the fluorophore was immediately cut off, generating only the 3′FAM monomer.

Example 12—Activity Profiling of Exonucleases: Directionality on Crystallisation Structure

To further support the directionality of the HL-ExoI I enzymes of the invention having 3′-5′ directionality, the HL-ExoI (Mv) was crystalised with ssDNA and the structure of the complex was determined.

Protein crystallisation was performed with a protein concentration of 5.4 mg/ml. The desalted 13mer oligonucleotide (dT13) was purchased from Sigma Aldrich and added to the protein in a 1.2 molar excess. To inhibit the degradation of the ssDNA by the exonucleaseI, 10 mM EDTA was added. The drops were set up automatically in a 96-well format in MRC 2 Well Crystallization Plates (Swissci, Hampton Research) with the Phenix (Art Robbin Instruments) using the sitting drop vapor diffusion technique. The drop size was 0.4 μl (0.2 μl+0.2 μl) and the volume of the reservoir solution was 60 μl. The crystallisation plates were incubated at 4° C.

A crystal co-crystallised with dT13/EDTA grew with 20.02% PEG MME 5000, 0.1 M Na-acetate pH 4.5 and 0.09 M Ca-acetate. The X-ray diffraction experiment was performed at the ESRF in Grenoble (France). The crystal diffracted to 2.5 Å resolution. Structure determination has been performed by molecular replacement with the E. coli ExoI (PDB: 1FXX) as the search model.

In the early rounds of refinement, electron density for the ssDNA became visible and all nucleotides could be fit in. The ssDNA binds with the 3′-end in the active site in a similar manner as seen for E. coli Exo I (Korada et al., Nucleic Acids Research, 2013, 41(11):5887-97) providing structural evidence for the HL-ExoI (Mv) 3′-5′directionality.

Example 13—Activity Profiling of Exonucleases: Rapid Inactivation of Certain HL-ExoI of the Invention at 80° C.

This experiment was performed to confirm that certain of the HL-ExoI of the invention could perform satisfactorily in a rapid PCR clean-up scenario. In this experiment, the thermal inactivation characteristics of the HL-ExoI under test at 80° C. were compared to two commercially available E. coli ExoI (ExoI A and ExoI B).

The activities of the HL-ExoI of the invention against ssDNA was calculated as described in Example 8, with the exception that the 1× assay mix consisted of 67 mM Glycin-KOH, pH 9.5, 63.5 mM NaCl, 9.2 mM MgCl₂, 10 mM DTT including ³H dA-labelled DNA. For the commercial E. coli ExoI, the activity was taken as that stated by the manufacturers. To mimic the set-up in a PCR clean-up setting, a post PCR buffer was used as the reaction buffer. The composition of the reaction buffer was 10 mM Tris-HCl pH 8.5 (25° C.), 50 mM KCl, 1.5 mM MgCl₂, DyNAzyme II (Thermo Fisher Scientific™, formerly Finnzymes™) and remnants of dNTPs (initially 200 μM of each before the PCR-reaction was run), remnants of primers (initially 200 nM of each primer before the PCR-reaction was run) and template.

Each reaction received 10 U ExoI, giving a final reaction volume of 7 μl. The experiment contained both an activity control for all ExoI, as well as a check for residual activity following heat incubation. For the activity control, reaction buffer, ExoI and 5 pmol substrate (GCTAACTACCACCTGATTAC; SEQ ID No 21) were mixed and incubated for 15 minutes at 30° C. followed by 95° C. for 20 minutes. For samples to be analysed with respect to inactivation at 80° C., substrate was added following 1 minute incubation at 80° C. and subsequent cooling. These samples were subsequently incubated for 15 minutes at 30° C. followed by 95° C. for 20 minutes. Following all incubation steps, TBE-Urea Sample buffer was added and samples were applied to a precast 20% acrylamide/7 M urea gel and run at 180 V for approximately 45 minutes.

Results are shown in FIG. 19. All Exo I had adequate activity in this assay. Only the HL-ExoI were inactivated following 1 minute incubation at 80° C., while the two commercially available E. coli Exo I showed adequate residual activity degrading 100% of the substrate. The HL-ExoI, but not the commercial ExoI, compatible with a rapid 5 minute PCR clean-up protocol.

To further compare the ease of inactivation of HL-ExoI (Sh) with that of two commercially available E. coli ExoI, 10 U of each ExoI was incubated for 1, 5, 10 or 20 minutes at 80° C. before cooling and addition of 5 pmol of a 5′ FAM labeled substrate (GCTAACTACCACCTGATTAC; SEQ ID No 21). The samples were further incubated for 15 minutes at 30° C. followed by 20 minutes at 95° C. Reaction set-up was otherwise identical to the above experiment, using the same post-PCR buffer as reaction buffer. An activity control was included for the three ExoI, and these samples were not subjected to heat incubation prior to incubation at 30° C. for 15 minutes. Residual activity was visualized on a Urea-PAGE gel as described above.

Results are shown in FIG. 20. Substantial residual activity was observed in both commercially available ExoI following heat treatment at 80° C. for nearly all time durations. In comparison, no residual activity could be detected in the samples treated with HL-ExoI (Sh) which had been treated at 80° C. for even a single minute.

Example 14—Demonstration of the Utility of HL-ExoI in a Rapid One-tube PCR Clean-up Prior to Nucleic Acid Sequencing

This example was performed to show functionality of certain HL-ExoI of the invention in a PCR clean-up situation. The experimental set-up was very similar to Experiment 4. As in Example 4, to the post PCR solution under test was added excess primers (10 pmol) following the PCR. However, unlike Example 4 only one PCR buffer (GoTaq, Promega) was used, the incubation temperature for samples treated with HL-ExoI was reduced from 37° C. to 30° C. and only the regular 30 minutes protocol was tested for ExoSAP-IT.

Prior to initiating the experiment, the activity of each HL-ExoI was calculated as described in Example 8, with the exception that the 1× assay mix consisted of 67 mM Glycin-KOH, pH 9.5, 63.5 mM NaCl, 9.2 mM MgCl₂, 10 mM DTT including ³H dA-labelled DNA. Samples treated with HL-ExoI received the amount of primers as stated above, before the addition of 2 μl premixed HL-ExoI (10-20 U/μl) and SAP (1.5 U/μl). Total volume for each clean-up reaction was 7 μl. Samples were incubated 4 minutes at 30° C. followed by 1 minute at 80° C. Samples were set up as triplicates

For comparison and positive control, samples were treated with a leading brand of enzymatic PCR clean-up; ExoSAP-IT™ (Affymetrix™). Samples treated with ExoSAP-IT were handled according to manufacturer protocol. Following addition of primers to the PCR solution, samples received 2 μl of the PCR clean-up reagent, giving a final volume of 7 μl. ExoSAP-IT-treated samples were incubated 15 minutes at 37° C. followed by 15 minutes at 80° C. Samples were set up as triplicates.

Negative controls were set up and these received the same amount of primers as treated samples. Instead of enzymatic clean-up solution, these samples received 2 μl water. Samples were set up as triplicates.

Example 4 should be referred to for more details.

Sequences were delivered to the DNA Sequencing core Facility at University of Tromsø for purification and sequencing using Applied Biosystems 3500xl Genetic Analyzer. Results were analyzed using the Sequence Scanner Software v2.0 (LifeTech)

Selected results are shown in FIG. 21, where the sequence plots are representative examples of the results from sequences spiked with reverse primers in the GoTaq PCR buffer. It was evident from the negative controls that lack of functional PCR clean-up strongly compromised the length and quality of the sequence. Samples treated with either ExoSAP-IT (30 minutes protocol) or HL-ExoI/SAP (5 minutes protocol) showed very good sequence quality. 

The invention claimed is:
 1. A method of nucleic acid amplification, said method comprising: (a) amplifying nucleic acid by a nucleic acid amplification reaction selected from the group consisting of a polymerase chain reaction (PCR), a ligase amplification reaction (LAR), a strand displacement amplification (SDA), a nucleic acid sequence based amplification (NASBA) and a loop-mediated isothermal amplification (LAMP) to produce a nucleic acid amplification product after a final amplification step of the nucleic acid amplification reaction and (b) subsequently contacting the nucleic acid amplification product with an exonuclease under conditions which permit the digestion of at least a portion of any single stranded DNA present in the nucleic acid amplification product and optionally then heating the mixture to inactivate said exonuclease, wherein said exonuclease has the amino acid sequence of SEQ ID NO:1 or an amino acid sequence which is at least about 85% identical thereto, wherein said exonuclease (i) is at least 90% irreversibly inactivated by heating at a temperature of about 55° C. for 10 mins in a buffer consisting of 10 mM Tris-HCl, pH 8.5 at 25° C., 50 mM KCl and 5 mM MgCl₂; (ii) has activity against double stranded DNA that is equal to or less than 15% of its activity against an equivalent amount of single stranded DNA under the same assay conditions; and (iii) has a 3′-5′ exonuclease activity.
 2. The method of claim 1, wherein said nucleic acid amplification product is the direct product of the final amplification step.
 3. The method as claimed in claim 1, wherein said exonuclease has the amino acid sequence of SEQ ID NO: 1 or SEQ ID NO:
 11. 4. The method as claimed in claim 2, wherein said exonuclease has the amino acid sequence of SEQ ID NO: 1 or SEQ ID NO:
 11. 